1. Field of the Invention
The present invention relates to a drive control device for drive control of plural ultrasonic motors.
2. Related Background Art
In driving a rotated member such as a rotary shaft or a rotary tube with plural ultrasonic motors, the torques generated by said ultrasonic motors may become mutually different at a certain rotating speed, because the relationship between the generated torque (output) and the rotating speed may be different from motor to motor.
This phenomenon will be explained in more detail with reference to FIG. 1, showing torsion moments generated in various parts of an output shaft of ultrasonic motors. A first ultrasonic motor 1 and a second ultrasonic motor 2 are mutually connected by a shaft 3, on which a load device 4 is further mounted. First and second torsion amount detecting devices 5, 6 mounted on said shaft 3 determine the torsion moments in the different parts of the shaft 3, and the outputs (torques) generated by said first and second ultrasonic motors 1, 2 can be determined by said torsion moments.
In the configuration shown in FIG. 1, if the generated torque as a function of the rotating speed is the same for both motors 1, 2, the torques T1, T2 generated by said motors 1, 2 become mutually equal. However, if the torque as a function of the rotating speed is different for the motors 1, 2, the torques T1, T2 generated by said motors 1, 2 become mutually different as indicated by broken lines, since they are connected by the same shaft 3 and have therefore a same rotating speed. The load torque TL is given by the sum of the generated torques T1 and T2.
Because of this phenomenon, the performances of both ultrasonic motors cannot be fully exploited.
Also in case either of the ultrasonic motors 1, 2 is scarcely rotated at a given frequency because of a significant difference in the performances of said motors 1, 2, such motor constitutes a load due to the pressed contact between the stator and the rotor, thus hindering improvement in the driving efficiency.
In order to overcome such a drawback, there has been proposed a configuration of providing the ultrasonic motors with respectively independent driving circuits and respectively regulating the frequencies of driving signals for said motors (cf. FIG. 7 in the Japanese Patent Laid-open Application No. 1-227669), but such regulation of the torques of the motors 1, 2 at substantially the same levels requires additional process steps, thus deteriorating mass producibility.
Also, in case the rotating speed has to be varied according to the situation of operation, the torques generated by the ultrasonic motors 1, 2 are mutually equal at the rotating speed at which the regulation was conducted, but may become mutually somewhat different at different rotating speeds because of the fluctuation in the motor performances.
Also, in case of connecting two or more ultrasonic motors of different performances, a motor of a lower power tends to be given an excessive load while a motor of a higher power tends to be given an insufficient load, if the same outputs are assigned to these motors. For this reason, the load has to be adjusted for each motor, according to its ability.
Also, in case of employing, as shown in FIG. 2, two or more ultrasonic motors 7, 8 as the driving sources for wheels 9, 10 of a movable member 11, a method of matching the outputs of said motors 7, 8 is effective when the movable member 11 proceeds linearly, but, when it turns in either direction, the outputs (generated torques and rotating speeds) of the motors 7, 8 have to be varied.